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New Trane XL15i short cycling heat

In January we had a new Trane XL15i heat pump professionally installed. I have noticed the system short cycling a few times while heating. The furnace fan stays on, the compressor fan stops then starts again within about 15 seconds. Sometimes the compressor fan will continue running for 5-10 minutes. Sometimes the compressor fan shuts off after a few seconds (usually followed by a loud bang). All the time the furnace fan is still running. Also I get large vibrations throughout the house when the system is doing this.

I checked the "Compressor off time" setting on the VisionPro IAQ thermostat. It was set to "0" so I changed it to the default of "5" minutes. This didn't change anything. The compressor fan still occassionally short cycles. When it does this it usually stops after about an hour and the Aux. Heat (LPG in our case) comes on.

The system normally runs fine with a 10-15 minute run time where both fans turn off simultaneously at the end. This odd behavior only happens now and then.

We are coming to the end of the heating season and I want to have this looked at before it warms up if something is wrong. Is this normal behavior?

Heat pump in the great white north!

I was born and raised in MA, so I am familliar with the winter conditions in the north east, I now live on the gulf coast in MS, this is where the heat pump is king, Take this with a grain of salt as I am not standing at your system, but it does sound like your condenser is going into defrost. the condenser fan will shut off and the reversing valve will switch the system back to "standard" flow, meaning it will run as an air conditioner, pumping hot gas through your outdoor coil, thawing it or bringing the outdoor stat back to temp. expect defrost to happen frequently up there as heat pumps typically lose their worth when temps drop below freezing. The noise is quite possibly a check valve having issues. Ask your local service company about a " magna-check " ( american standard product, so therefor a trane product. Hopefully this helps your company a bit, I didnt know what i was looking at my first heat pump system, so i can imagine those feller's up north are a bit taken by it. Do you have a Trane or American standard coil, or is it the original lennox coil?

Last edited by ACRDave; 03-30-2010 at 04:21 PM.
Reason: Caught another good cause for issues.

The service guy just left. The heat pump was cutting out due to high pressure. He is coming back out to replace indoor & outdoor TXV valves, dryer and do a flush and recharge. He was actually really knowledgable. He says he'll call Trane tomorrow to check on any recalls/problems on this model.

I'm willing to bet this happened because of the problems I had with the original install. The 1st installers spliced together 3 linesets without running nitrogen. They also installed the wrong unit. So I had the service manager come back out and install the right unit and a new lineset while running nitrogen. BUT - they didn't replace the coil. So whatever crud was in there may have gummed up one or both of these valves. Anyhow it is all under warranty. I'll let you know later in the week how it turns out.

Bad TXV, unless it has a horrible restriction like at the indoor coil, doesn't cause high pressure. Overcharge or low airflow does. Bad TXV would cause low pressure. Both wouldn't need changing, only the culprit would.

He did say he suspected the indoor TXV mainly (which seems to backup my theory about leftover crud in the coil). The rest of your post went over my head. Would the things he is going to do resolve this regardless of the 'culprit'?

I agree with Baldloonie, sounds like overcharge or low air flow. Could also be non-condensibles in the system {air}. In my experiance a bad TXV will act up all the time, not on an intermitant basis like what I understand you have ie sometimes it runs 10-15 minutes just fine, other times it doesn't. I would be supprised if the TXV fixes anything, but in the process if the charge gets corrected or N/Cs get taken out of the system it will appear that it fixed it.

Does the furnace come on when the system goes into defrost? It could be that when the unit goes into defrost the furnace comes on but doesn't shut down soon enough after so that when the heat pump goes back into heating the heat from the furnace causes the pressures to go up and the unit goes out on high pressure.

Sounds like the service guy saw something he didnt like, possibly N/C, wrong txv valves, I wonder if there was a sight glass/moisture indicator in line. He probably knows the history of the unit or something we can't see over the net.

Does the furnace come on when the system goes into defrost? It could be that when the unit goes into defrost the furnace comes on but doesn't shut down soon enough after so that when the heat pump goes back into heating the heat from the furnace causes the pressures to go up and the unit goes out on high pressure.

He had the gauges on and we actually saw it climb to 600 then the compressor fan shutoff until the pressure dropped (about 15 seconds) then the compressor fan would come back on. It didn't do this every cycle. Sometimes it would run about 10 minutes just fine. Then both the furnace fan and compressor would shutdown as it should.

Sounds like the service guy saw something he didnt like, possibly N/C, wrong txv valves, I wonder if there was a sight glass/moisture indicator in line. He probably knows the history of the unit or something we can't see over the net.

This system is 2 months old and has never been serviced since installation. As far as I know there is no sight glass/moisture indicator inline. I gave him the installation history and the likelihood of the coil having leftover oxidation from the first misinstall. When he heard that he decided to just replace all parts that might be affected and flush the whole system. Hopefully this will fix the issue.

My one worry is this brand new system has been doing this for 2 months. Could this rapid on/off of the compressor motor have damaged it or any other parts? It would happen for an hour at a time until the thermostat decided to use Aux. heat.